Thermal swing adsorption (TSA) systems for the removal of contaminant from a gas stream are taught in the art. TSA systems generally comprise a repeating cycle of steps including:                (i) contacting the gas stream with an adsorbent selective for the retention of a contaminant in order to adsorb at least a portion of the contaminant from the gas stream wherein this step (i) is conducted at an initial temperature;        (ii) heating the adsorbent to a regeneration temperature in order to desorb at least a portion of the contaminant adsorbed in step (i); and        (iii) cooling the adsorbent to the initial temperature before starting a new cycle.        
The regeneration temperature generally ranges anywhere from 40° C. to 400° C., but once selected, generally remains generally constant from cycle to cycle. See for example U.S. Pat. Nos. 5,531,808, 5,689,974, 5,906,675, 6,106,593, 6,273,939, and EP 1226860.
The adsorbent bed in TSA systems typically includes a layer of desiccant (such as silica gel or alumina) to remove water since, even at ppm levels, water adversely impacts the capacity of the adsorbent to adsorb contaminants. Notwithstanding the use of desiccants, water ingress into the adsorbent remains a problem in TSA systems. This is especially true just prior to start-up when the adsorbent is initially loaded in wet ambient air, or during a plant upset in which water breaks through the desiccant layer and into the adsorbent layer. In the past, this type of situation has required that the adsorbent be discarded and fresh adsorbent loaded.
The present invention addresses the problem of water ingress by periodically heating the adsorbent to a second regeneration temperature greater than the first regeneration temperature. The present invention is particularly useful where the TSA adsorbent utilized is particularly sensitive to water, such as where the adsorbent comprises a zeolite molecular sieve.
Pressure swing adsorption (PSA) systems are also taught in the art for the removal of contaminant from a gas stream. PSA systems generally comprise a repeating cycle of steps including:                (i) passing the gas stream through a vessel containing an adsorbent selective for the retention of a contaminant in order to adsorb at least a portion of the contaminant from the gas stream wherein this step (i) is conducted at an initial elevated pressure;        (ii) depressurizing the adsorbent-containing vessel in order to desorb at least a portion of the contaminant adsorbed in step (i); and        (iii) repressurizing the adsorbent-containing vessel to the initial elevated pressure before starting a new cycle.        
It can be seen that whereas regeneration of adsorbent in TSA systems is accomplished by a temperature swing, regeneration of adsorbent in PSA systems is accomplished by a pressure swing. Consequently, PSA systems include no cyclical heating of the adsorbent.
U.S. Pat. No. 5,931,022 teaches a PSA system which includes non-cyclical heating of the adsorbent. In particular, U.S. Pat. No. 5,931,022 teaches periodic heating of the adsorbent to temperatures between 50 and 300° C. to address water ingress into the adsorbent. In the case of U.S. Pat. No. 5,931,022, an adsorbent comprising alumina is utilized to remove CO2. The skilled practitioner will appreciate that alumina is easy to dehydrate, at least compared to the zeolite molecular sieve type adsorbent for which the present invention is particularly suited.
U.S. Pat. No. 4,481,018 teaches a PSA system which utilizes an X type zeolite for N2 removal. This patent recognizes the importance of regeneration gas flow rate to ensure good N2 capacity of zeolites, typically Ca exchanged X zeolites. Table V in this patent shows the importance of regeneration gas flow rate or contact time on the N2 capacity of CaX. At a regeneration gas contact time of 9 seconds (0.15 minutes), the N2 Henry's law constant on CaX was 2.4 mmole/g/atm. When the regeneration gas contact time was increased to 27 seconds (0.45 min), the N2 Henry's law constant was decreased 33% to 1.6 mmole/g/atm. U.S. Pat. No. 4,481,018 does not teach the effect of regeneration gas contact time on CO2 capacity as per Example 4 herein.